At a rally last week in Dallas, reporters observed hundreds of supporter allowed into the event without undergoing the same screening checks that are standard at smaller Obama events. In an effort to accommodate the 17,000-strong crowd, Secret Service agents ordered local police to stop searching each supporter individually who entered the event — this was at least the third time reporters have noted such an adjustment at large events in recent months.

"It looks like Obama is the Democratic front-runner, plus he's an African-American, so I'm not surprised by the protection, at all. In fact, I think it's good that they're tightening security," said David Katz, CEO of the Global Security Group, and a former FBI instructor and DEA special agent.

But the Obama campaign insists that no corners are being cut. "He has the best protection provided in the world. Every precaution is taken for an event," said a senior advisor to the Obama campaign.

Secret Service spokesman Ed Donovan would not discuss these specific events or any other details about the way Obama is being protected, however, he said, ""Many of the security measures that we utilize are not visible to the general public. And unfortunately we cannot discuss the other security methods that we utilize."

Even with large turnout at political rallies, the desire to move the crowd into an event does not override security, the Obama campaign says. "The Secret Service would never put anybody in a position of minimizing security for any other goal," the senior campaign advisor said. "They (the Secret Service) have one job to do and they do it 100 percent without cutting any corners."

"It is of the utmost importance now that the Secret Service provides the most rigorous protection for Sen. Obama," said ABC News consultant Fawaz Gerges, author of numerous books on national security and the terrorist threat. "It is a serious concern for many Americans in light of the historical precedents with Martin Luther King and President John F. Kennedy and Robert Kennedy."

To reporters traveling with the candidate, it seems there are visible signs that security around the senator is increasing. On a few recent occasions, armed snipers were spotted on rooftops, including at an outdoor rally in Wilmington, Del., and another in Clemson, S.C., in January. Reporters traveling with the candidate have noted an increase in protective measures, with background checks performed on reporters who have already been with the campaign on a daily basis for months.

"The most difficult thing for the Secret Service is to try to identify and to stop the person who's a threat," added Katz. "A lot these receiving lines, at these different venues, have areas with a lot of people pretty close to him, as with all these high-profile candidates. And I think that's probably why they're tightening it up — more people in line, more people in the crowd, more people around the senator, just to watch out for that type of tactic."